The individual cell appears to be entirely self-sufficient; it captures food, digests it, grows to maximum size and then divides (Fig. 1). The original individual cell no longer exists, although it has not died; its protoplasm is now distributed between two daughter cells. In the same way these cells grow old in turn, divide into two each, and so on apparently in endless succession of cell generations. Obviously if this could keep on indefinitely there would be a basis for the view that Protozoa are immortal. They do not keep this up, however, but there comes a time when the nature of the protoplasmic make-up changes, and processes similar to fertilization in Metazoa supervene. In the great majority of parasitic protozoa and in most free-living Fig. 2.Original. forms that have been studied in culture, there comes a period when certain cells of the race, or specialized parts of the protoplasm of all of the cells of the gig race, undergo marked changes. different from any vegetative phase, and reorganization of the old individual or formation of new ones is the outcome. This result is brought about by conjugation or the union of two cells in more or less complete coalescence, during which an interchange and mixture of germ plasms is accomplished (Fig. 2).
In some of the best-known forms of Protozoa, notably in Paramecium caudatum, the conditions are quite different from those of the majority of protozoa and too many generalizations have been made upon the comparatively rare phenomena which are manifested in this "slipper animal" and its immediate allies. In the conjugation of Paramecium two individual cells unite very much as do Blepharisma cells. In each individual there are two types of nuclei, one, a large macronucleus, plays no part in the fertilization process, but, sooner or later, disintegrates and dissolves in the cell. The other is a minute micronucleus, which divides three successive times, giving rise to a number of micronuclei, which, with the exception of two germ nuclei, also disintegrate and dissolve in the cell. These two germ nuclei are sexually differentiated, one is smaller than the other, and migrates into the other cell of the pair, there uniting with the stationary larger form of nucleus (Fig. 3). Thus there is a mutual fertilization of the two